In conventional analog/digital (A/D) converters, there is no precondition on characteristics of the analog signal to be converted, apart from the criterion that the frequency, with which the analog signal can change, must be below twice the critical limit or sampling frequency of the A/D converter. The investment required to produce an A/D converter depends upon the precision and speed desired in the digital conversion.
"Precision" in this context means the number of bits which, after the conversion, are used to represent the original sampled analog value. In the following discussion, an 8-bit data word representation is considered imprecise, while a 14-bit digital signal value is considered very precise. "Speed" in this context is measured by the time which the converter needs to convert an analog signal into, for example, an 8-bit data word, or to convert a 14-bit-wide signal value into an analog signal. The conversion is considered slow if the response time or pull-in time of the converter approximates a quarter-period of the carrier frequency of the FM signal. The conversion is considered fast whenever the response time of the converter is short or small relative to the carrier period.